1. Field of the Invention
The present invention relates to a calibration apparatus for vacuum gauge, and more in particular, to a calibration apparatus for vacuum gauge, which can be used not only for an 1 Torr level vacuum gauge, a 10 Torr level vacuum gauge, but also for a 100 Torr level vacuum gauge also.
2. Background of the Related Art
Recently, researches and developments about measurement of small pressure have been performed vigorously due to developments in environmental industries and process industries. In general, the measurement of small pressure refers to the measurement of pressures falling in a range below several thousands of Pa, that is, it refers to the measurement of pressures falling in the range from one-several hundredths to one-several thousandths of the atmospheric pressure, in view of the fact that the atmospheric pressure is about 100 kPa (100,000 Pa).
A measuring gauge for small pressure using mercury or oil has been developed or is under international development. The vacuum gauge for measuring small pressures can be divided into 1 Torr type vacuum gauge, 10 Torr type vacuum gauge, and 100 Torr type vacuum gauge depending on the commercial applications, and such vacuum gauge occupies 95% of all vacuum gauges.
Meanwhile, a laser interference type or an ultrasonic interference type mercury pressure gauge has been employed in calibration of such vacuum gauge for measuring small pressures. However, in general such reference pressure gauge has not been employed in general calibration body and test body due to its complexity of device and aversion for mercury. Accordingly, up to date, after any one vacuum gauge has been calibrated, a next vacuum gauge is calibrated with help of the previous calibrated vacuum gauge. However, such method increases uncertainty of the gauge, and decrease calibration efficiency because continuous periodical calibration (for instance, every six month) for reference gauge is necessarily in demand.
In addition, according to the recent technologies developed until now, the vacuum gauge for 10 Torr level vacuum has been calibrated by the calibration apparatus of 10 Torr type vacuum gauge, and the vacuum gauge for 100 Torr level vacuum has been calibrated by the calibration apparatus of 100 Torr type vacuum gauge respectively. Difficulties arising from the measurement and calibration have been increased, as the pressure to be measured becomes lower (for instance, 1 Torr).
FIG. 1 is a view for showing whole systematic construction of a calibration apparatus applicable to the present invention and to the conventional art. As shown in FIG. 1, the conventional calibration system for vacuum gauge may comprise a calibration apparatus for vacuum gauge 100, a gas supply device, a monitoring pressure gauge 560, and a gas discharge device.
The gas supply device comprises a gas supply valve 542, a volume variable valve 544, and a first vacuum pump 550, and the like for supplying predetermined pressure via a gas supply pipe 540 to a piston which will be described below. Especially, the first vacuum pump 550 includes a trap 552 for blocking back-streaming of oil vapor in addition to a pump valve 554 as rotary type pump is employed for it. Also, a portion of the gas supply pipe 540 is branched to be connected to the monitoring pressure gauge 560.
The gas discharge device operates to form a vacuum inside the calibration apparatus 100 of the vacuum gauge, and includes a second vacuum pump 580, a trap 582 and a blocking valve 584 having identical construction. Also, a needle valve 574 and a vent valve 572 are connected to one end of a gas discharge pipe 570.
Hereinafter, conventional art of the calibration apparatus for vacuum gauge shown in FIG. 1 will be described. FIG. 2 is a view for showing schematic construction of the conventional calibration apparatus 10 for vacuum gauge. As shown in FIG. 2, a pressure vessel 20 is sealed from the outside, and is provided with a piston 40 and a cylinder 42 constituting a force balance type pressure gauge within it. Also, the weight sets 30 are arranged above the piston 40, and a lower portion of the piston 40 is connected to the gas supply pipe 540.
In such calibration apparatus 10 for vacuum gauge, equilibrium state can be achieved by supplying gas via the gas supply pipe 540 after loading all the weight sets 30 on the piston 40. The piston 40 is maintained at a raised state after it has been come up by means of the equilibrium of upper and lower forces.
In this state, the monitoring pressure gauge 560 can be calibrated by determining whether the indicated value of the monitoring pressure gauge 560 conforms to the measured pressure value Pi based on following formula (1):Pi=F/A+Pr  (1)
Here, Pi represents measured pressure value, F represents total downward force obtained by adding the mass of the piston 40 and that of the tare weight, A represents cross-sectional area of the piston 40, and Pr represents reference pressure inside the pressure vessel 20.
The reference pressure Pr is the pressure formed around the piston, and becomes to be vacuum at the time of measuring the absolute pressure and becomes to be atmospheric pressure at the time of measuring the gauge pressure. The reference pressure Pr is maintained at vacuum or atmospheric pressure and is regarded as a predetermined value during the measurement. However, as F represents gravitational load obtained by adding the mass of the piston and that of the weight sets in formula (1), it always has a minimum value exceeding the mass of the piston. Accordingly, with regard to the measured pressure Pi, minimum pressure (normally, several kPa) exists, which corresponds to the minimum value described above, and therefore, it is impossible to measure pressures below the above pressure.
A method for variable residual pressure, which is denoted by following formula (2), is suggested as a substitute for the above described measuring method, in which it is possible to cope with the change of the load due to change of the weight sets by maintaining the pressure Pm below the piston to be uniform and varying the reference pressure Pr′ around the piston:Pm=F/A+Pr′  (2)
Here, Pm represents the pressure monitored below the piston, F represents the gravitational load obtained by adding the mass of the piston and that of the weight sets, A represents cross-sectional area of the piston, and Pr′ represents variable reference pressure around the piston.
However, such method has not also been employed in practical, because it is difficult to change the weights located within a vacuum chamber effectively without breaking such low pressure as it is under the vacuum or reference pressure.
Furthermore, calibration of the vacuum gauge is difficult and is not economical, because respective vacuum gauges such as 1 Torr type, 10 Torr type and 100 Torr type vacuum gauge should be calibrated by means of respective calibration apparatus for the vacuum gauge.